Cove‐Edged Hexa‐peri‐hexabenzo‐bis‐peri‐octacene: Molecular Conformations and Amplified Spontaneous Emission

Abstract The bottom‐up synthesis of an unprecedentedly large cove‐edged nanographene, hexa‐peri‐hexabenzo‐bis‐peri‐octacene (HBPO), is reported in this work. Chiral high‐performance liquid chromatography and density functional theory (DFT) calculations revealed multiple conformations in solution. Two different molecular conformations, “waggling” and “butterfly”, were found in crystals by X‐ray crystallography, and the selectivity of conformations could be tuned by solvents. The optoelectronic properties of HBPO were investigated by UV/Vis absorption and fluorescence spectroscopies, cyclic voltammetry, and DFT calculations. The contorted geometry and branched alkyl groups suppress the aggregation of HBPO in solution, leading to a high fluorescence quantum yield of 79 %. The optical‐gain properties were explored through transient absorption and amplified spontaneous emission spectroscopies, which enrich the choices of edge structures for potential applications in laser cavities.


General methods
All reactions with air-or moisture-sensitive compounds were carried out under argon atmosphere using standard Schlenk line techniques. Unless otherwise noted, all starting materials were purchased from commercial sources and used without further purification. All other reagents were used as received.
Column chromatography was conducted with silica gel (grain size 0.04 -0.063 mm) and thin-layer chromatography (TLC) was performed on silica gel-coated aluminum sheets with an F254 indicator.
Nuclear magnetic resonance (NMR) spectra were recorded in methylene chloride-d 2 , chloroform-d, or THF-d 8 on AVANCE 300 MHz and AVANCE 500 MHz Bruker spectrometers. The chemical shift was recorded in ppm and the following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, dd = doublet of doublets. Abbreviations of solvents: DMF = N,Ndimethylformamide, DCM = dichloromethane, THF = tetrahydrofuran. High-resolution mass spectra (HRMS) using atmospheric pressure chemical ionization (APCI) were recorded on a MicrOTOF-QII instrument. UV-Vis absorption spectra were taken on a Perkin-Elmer Lambda 900 spectrophotometer using a 10 mm quartz cell. The fluorescence quantum yield () was measured using Nile blue A perchlorate (in ethanol under air,  = 0.27) as a reference. 1 Photoluminescence spectra were recorded on an FL3095SL spectrometer (J&M TIDAS 9.5, Germany). Cyclic voltammetry (CV) was performed on a WaveDriver 20 Bipotentiostat/Galvanostat (Pine Instruments Company) and measurements were carried out in DCM containing 0.1 M n-Bu 4 NPF 6 as supporting electrolyte at room temperature (scan rate: 100 mV/s). A glassy carbon electrode was used as a working electrode, a platinum wire as a counter electrode, and a silver wire as a reference electrode. High-performance liquid chromatography (HPLC) analysis was performed on an Agilent 1200 Series equipped with the following modules: quaternary pump (G1311A 1100), manual sample injector (Rheodyne 7725i), column thermostat (G1316A 1200), and DAD detector (G1713B1200). The chiral column used was Daicel Chiralpak IE analytical column (4.6 × 250 mm) packed with amylose tris-(3,5-dichlorophenylcarbamate) immobilized on silica gel (5 μm).
The column temperature was set at 20 ℃ and the flow was constant during the operation. Density functional theory calculations were conducted at the B3LYP/6-31G(d,p) level using Gaussian 09 software package. 2

Synthetic details
Compound 1 was synthesized by following the same procedure as reported before. 3 Compound 1 (0.76 g, 2 mmol), 4-tert-butylphenylboronic acid (1.07 g, 6 mmol, 3 equiv.), and potassium phosphate (2.54 g, 12 mmol, 6 equiv.) were mixed in a solution of DMF (15 mL) and water (2 mL). After the reaction mixture was bubbled under nitrogen for 15 mins, catalyst [1,1′-bis(diphenylphosphino) ferrocene]dichloropalladium(II) complex with DCM (164 mg, 0.2 mmol, 10 mol%) was added in one portion. The mixture was heated to 90 ℃ and stirred overnight. After that, the black mixture was cooled to room temperature, 100 mL of ammonium chloride solution was added. The aqueous phase was extracted by ethyl acetate (3×50 mL). The combined organic phase was washed with brine and water and dried over sodium sulfate. After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using hexane as eluent, affording 2 as a colorless solid (0.78 g, 80%). 1  To a solution of compound 2 (4.87 g, 10 mmol) in methanol (59 mL) and DCM (59 mL) at 0 ℃, bromine (1.53 mL, 30.1 mmol) was added slowly. The mixture was stirred overnight at room temperature. The reaction was quenched by saturated aqueous sodium sulfite solution and extracted with DCM (2×50 mL).
The combined organic phase was washed with brine and water and dried over sodium sulfate. After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using hexane as eluent, affording 3 as a colorless solid (3.5 g, 70%). 1  Compound 3 (3.70 g, 7.4 mmol), 4-tert-octylphenylboronic acid pinacol ester (3.05 g, 9.62 mmol, 1.3 equiv.), and potassium carbonate (5.11 g, 37 mmol, 5 equiv.) were mixed in a solution of THF (333 mL) and water (37 mL). After the reaction mixture was bubbled under nitrogen for 15 mins, catalyst tetrakis(triphenylphosphine)palladium(0) (212 mg, 0.185 mmol, 2.5 mol%) was added in one portion. The mixture was heated to 80 ℃ and stirred overnight. The reaction mixture was extracted by diethyl ether (2×50 mL). The combined organic phase was washed with brine and water and dried over sodium sulfate.
After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using hexane as eluent, affording 4 as a colorless solid (1.95 g, 44%). 1  Compound 6 was synthesized by following the same procedure as reported before. 4 Compound 6 (658 mg, 1 mmol) and copper iodide (38 mg, 0.2 mmol, 20 mol%) were mixed in a solution of triethylamine (30 mL). After the reaction mixture was bubbled under nitrogen for 15 mins, catalyst bis(triphenylphosphine)palladium(II) dichloride (80 mg, 0.11 mmol, 11 mol%) was added in one portion.
Then trimethylsilylacetylene (0.7 mL, 5 mmol, 5 equiv.) was added slowly to the reaction mixture, which was further stirred overnight at 50 ℃. The reaction mixture was then filtered through Celite. After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using DCM/hexane (1:9) as eluent, affording 7 as a colorless solid (400 mg, 67%). 1  equiv.), and potassium phosphate (795 mg, 3.75 mmol, 3 equiv.) were mixed in a solution of toluene (12.5 mL), ethanol (1.25 mL), and water (1.25 mL). After the reaction mixture was bubbled under nitrogen for 15 mins, catalyst tetrakis(triphenylphosphine)palladium(0) (115 mg, 0.1 mmol, 8 mol%) was added in one portion. The mixture was heated to 105 ℃ and stirred overnight. The reaction mixture was extracted with diethyl ether (2×50 mL). The combined organic phase was washed with brine and water and dried over sodium sulfate. After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using DCM/hexane (1:9) as eluent, affording 8 as a yellow solid (620 mg, 77%).
After the reaction mixture was bubbled under nitrogen for 15 mins, catalyst tetrakis(triphenylphosphine)palladium(0) (230 mg, 0.2 mmol, 50 mol%) was added in one portion. The mixture was heated to 95 ℃ and stirred overnight. The reaction mixture was extracted by diethyl ether (2×50 mL). The combined organic phase was washed with brine and water and dried over sodium sulfate.
After the solvents were removed by rotary evaporation, the residue was purified by silica gel chromatography using DCM/hexane (1:9) as eluent, affording 12 as a yellow solid (740 mg, 96%). 1  Compound 12 (190 mg, 0.1 mmol) and indium(III) chloride (6.6 mg, 0.03 mmol) were dissolved in mesitylene (20 mL) under nitrogen atmosphere. The mixture was heated overnight at 150 °C. After cooling to room temperature, the solvent was removed under vacuum. The mixture was extracted with DCM (3×30 mL) and sodium carbonate solution (30 mL). The combined organic layer was dried over sodium sulfate. The solvent was removed under vacuum and the residue was purified by column chromatography (silica gel, hexanes/DCM = 9/1) to afford compound 13 as an orange color solid (171 mg, 90%). 1   A solution of compound 13 (30 mg, 0.015 mmol) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 60 mg, 0.27 mmol) in dry degassed DCM (10 mL) was stirred at 0°C for 15 mins. Then trifluoromethane sulfonic acid (0.11 mL) was added in one portion. The mixture was stirred overnight at room temperature and subsequently quenched with Et 3 N (1 mL). After 30 mins, the mixture was extracted with DCM (2×50 mL) and aqueous sodium bicarbonate (50 mL). The combined organic portion was dried with sodium sulfate. The solvent was removed under vacuum and the residue was washed with methanol (30 mL) to remove the remaining DDQ. After filtration, the precipitated solid was further washed with hexane to remove partially oxidized compounds. The precipitate was purified by column chromatography (silica gel, toluene) to afford the target compound HBPO as a magenta solid (14 mg, 46%). 1   Using the ferrocene/ferrocenium couple as an external reference, the electrochemical properties of HBPO were studied by cyclic voltammetry (CV). Three oxidation waves with half-wave potentials E 1/2 ox at 0.15, 0.60, and 1.06 V and two reduction waves with half-wave potentials E 1/2 red at -1.68 and -2.01 V were observed. The HOMO and LUMO energy levels were estimated from the onset of the first oxidation/reduction wave potential to be 4.92 eV and 3.17 eV, respectively, consistent with the molecular orbital energy levels based on DFT calculations (4.33 and 2.23 eV). Therefore, the electrochemical energy gap was determined to be 1.75 eV.

DFT calculations
Theoretical calculations were performed with the Gaussian09 rev. D program suite. 2 All calculations were carried out using the density functional theory (DFT) method with Becke's three-parameter hybrid exchange functionals and the Lee-Yang-Parr correlation functional (B3LYP) employing the 6-31G(d,p) basis set for all atoms. 5 The geometry of HBPO was optimized under a Pople basis set 6-31G (d,p). 6 NICS values were calculated using the standard GIAO procedure. 7 ACID plot was calculated by using the method developed by Herges. 8      DFT calculations were performed to evaluate the isomerization process of HBPO. As demonstrated in Figure S7, there is one plausible pathway, involving two transition states (TSs) and one metastable helical conformation. The simplified HBPO without branched alkyl substituents was also optimized to calculate the isomerization barriers for comparison.

Transient Absorption Spectroscopy and Amplified Stimulated Emission
Characterization of HBPO HBPO solutions and films for TA spectroscopy and ASE measurements were prepared as follows. HBPO solutions were obtained by diluting HBPO in anhydrous chloroform at a concentration displaying high fluorescence quantum yield (Optical density ~ 0.1 at 602 nm). For this purpose, 1 mg of HBPO was dissolved in 5 mL of anhydrous chloroform and diluted several times. Before any dilution or measurement, a mild sonication was applied to the sample. All steps of sample preparation were performed under an inert atmosphere. HBPO/PS blend films were prepared with 1.24, 2,43, and 5 wt% HBPO contents.
Accordingly, 1 mg of HBPO was dispersed in 2 mL of chloroform and sonicated mildly. In parallel, a solution containing 60 mg of PS in 1 mL of anhydrous chloroform was prepared and stirred overnight.
Both master solutions were subsequently mixed to obtain the desired HBPO wt% content. Finally,

X-ray Crystallographic data
The single crystals of compound HBPO suitable for X-ray analysis were obtained by slow diffusion of methanol into the toluene solution of HBPO. The X-ray diffraction data were collected at 120 K on a STOE IPDS 2T diffractometer by using graphite monochromated Cu-Kα radiation.
Cambridge Crystallographic Data Centre and the data can be obtained free of charge via www.ccdc.cam.ac.uk/structures. Crystallographic data with CCDC number 2042061 and 2141365. Remark molecule has pseudo-Ci symmetry, tbutyl and tert-octyl groups are disordered, one mesitylene is half occupied at centre of inversion 7. NMR and Mass spectra Figure S14. 1 H NMR spectrum of compound 2 (300 MHz, methylene chloride-d2).                          (TS1 and TS2) of HBPO, Dibenzoperi-octacene, and peri-Octacene.